Conventional lithium ion secondary batteries include a plate-shaped positive electrode, a separator, and a plate-shaped negative electrode. Such a battery is formed by using, as an electrolyte solution, an organic solvent such as ethylene carbonate (EC) or dimethyl carbonate (DMC) in which a lithium salt such as LiPF6 is dissolved. In general, an aluminum foil to which a lithium metal oxide is applied is used as a positive electrode current collector, and a copper foil to which a carbon material is applied is used as a negative electrode current collector. Generally speaking, an organic solvent in which a lithium salt is dissolved is used as an electrolyte. A microporous polypropylene or polyethylene film having a thickness of 30 to 80 μm is used as a separator.
Conventional secondary batteries in which an aqueous solution is used as an electrolyte solution include a plate-shaped positive electrode, a separator, and a plate-shaped negative electrode. Generally speaking, in the case of an alkaline secondary battery, an aqueous solution such as a potassium hydroxide aqueous solution, or a potassium hydroxide aqueous solution in which lithium hydroxide is dissolved, is used as an electrolyte solution, and in the case of a lead battery, dilute sulfuric acid is used as an electrolyte solution.
In ordinary alkaline secondary batteries such as nickel metal-hydride batteries and nickel-cadmium batteries, an electrode having a thickness of approximately 0.65 to 0.8 mm is used as a high-capacity electrode, and an electrode having a thickness of approximately 0.3 to 0.5 mm is used as a high-power electrode.
Well-known methods used for fabricating positive electrodes of these alkaline secondary batteries include: a method in which a positive electrode active material is impregnated into a base material (i.e., sintering process); and a method in which a paste containing an active material is filled into a foamed nickel base material (i.e., paste process). In a method commonly used for fabricating negative electrodes of these alkaline secondary batteries, a paste process is performed where a current collector having a two-dimensional structure, such as a perforated metal, is coated with a paste containing an active material and then pressurized. A sintered body that is obtained by sintering carbonyl nickel to a perforated metal, or a porous nickel foam obtained by removing a resin from nickel-plated resin foam through incineration, is widely used as a positive electrode current collector. Although there are publicly known porous bodies having irregularity that are formed through mechanical processing, such porous bodies have not been developed to a practical level. A method commonly used for fabricating electrodes of lead batteries is a paste process. Electrodes used in lead batteries have a greater thickness than that of electrodes used in alkaline secondary batteries.
In alkaline secondary batteries, a polyamide nonwoven fabric or a hydrophilically-processed polyolefin-based nonwoven fabric, having a thickness of approximately 80 to 200 μm, is commonly used as a separator. In lead batteries, a porous body such as paper, porous polyolefin plate, or fiberglass cloth is used as a separator. Generally speaking, lead batteries are required to contain a large amount of sulfuric acid which is directly involved in charge/discharge reactions. Therefore, a porous body used in lead batteries has a greater thickness than that of a porous body used in alkaline secondary batteries.
Conventional electric double layer capacitors include plate-shaped positive and negative electrodes which are both formed of activated carbon having a large surface area. An electrolyte solution used in such electric double layer capacitors may be either an aqueous electrolyte solution or a nonaqueous electrolyte solution. A sulfuric acid aqueous solution containing approximately 30 wt % of sulfuric acid, or an aqueous solution of potassium hydroxide, is used as an aqueous electrolyte solution. The use of an aqueous electrolyte solution is advantageous from the viewpoint of high-rate charging/discharging (rapid charging/discharging) since an aqueous electrolyte solution has greater ion conductivity than that of a nonaqueous electrolyte solution. However, in the case of an aqueous electrolyte solution, the operating voltage is 1.2 V, which is low, because the operating voltage is limited due to the decomposition potential of water. On the other hand, an electrolyte solution that is obtained by dissolving a salt containing tetrafluoroboric acid or an ethyl group (e.g., tetraethylammonium or tetraethylphosphonium) into an organic solvent such as propylene carbonate is used as a nonaqueous electrolyte solution. Such a nonaqueous electrolyte solution has a stable potential range wider than that of aqueous electrolyte solutions, and therefore, is applicable to capacitors that operate at high voltages of 2 to 4 V.
In ordinary batteries, a solid active material is used as a positive electrode and as a negative electrode. However, in some batteries, gaseous oxygen is used as a positive electrode active material. Such a battery is called an air battery. Coin-shaped zinc-air batteries, in which zinc is used for a negative electrode, are already in practical use. However, these zinc-air batteries are primary batteries. Developments in air secondary batteries with great energy density such as lithium-based batteries have been actively conducted. Such an air secondary battery includes a cathode that serves smooth gas supply and that serves to prevent leakage and volatilization of an electrolyte solution. The cathode used here is formed of a carbon material in which polytetrafluoroethylene (PTFE) is mixed. Discharging progresses when oxygen that has passed through the cathode reacts with the electrolyte. Here, the reaction progresses smoothly if three-phase boundaries, at which the solid phase (cathode material), the liquid phase (electrolyte solution), and the gas phase (oxygen) are in contact with each other, suitably exist.
Known primary batteries, which are not rechargeable after discharging, are dry batteries widely used in small-sized handheld devices such as watches and flashlights. Among the dry batteries, conventional dry batteries are: manganese dry batteries in which manganese dioxide and zinc are used for a positive electrode and a negative electrode, respectively, and a zinc chloride aqueous solution is used as an electrolyte solution; and alkaline manganese batteries in which a potassium hydroxide aqueous solution to which zinc chloride is added is used as an electrolyte solution. Alkaline manganese batteries have greater energy density than that of manganese dry batteries. However, alkaline manganese batteries have high self-discharge. These primary batteries are considered to be unsuitable for applications because internal resistance is high and power consumption is great. Other known primary batteries include silver oxide batteries, mercury batteries, zinc-air batteries, and lithium batteries.
The inventors of the present invention have proposed a battery structure, the conception of which is completely different from that of the above-described conventional electrode assembly which includes a positive electrode, a separator, and a negative electrode. In the proposed battery, a fibrous body having electron conductivity is used as a current collector (see Patent Literature 1). Patent Literature 1 discloses a battery which is particularly intended to realize high power.
Patent Literature 2 discloses a cord-like structure in which: one of an elongated negative electrode member and an elongated positive electrode member, each of which has an electrode active material formed on its outer periphery, is used as a core; the other electrode member is provided around the outer periphery of the core in a concentric manner, with a polymer solid electrolyte disposed between the core and the other electrode member; and these electrode members are sealed by external cladding. Patent Literature 2 discloses a structure which is fundamentally the same as the structure of a conventional Leclanché cell. Specifically, in a dry battery, a positive electrode member is disposed at the center, a negative electrode member is disposed at a peripheral part, and an electrolyte is disposed between these electrode members; and the overall shape is cylindrical. Patent Literature 2 proposes a cord-like structure in which a solid electrolyte is used and which is flexible in its entirety.
Patent Literature 2 does not disclose a specific electrode thickness. However, since the cord-like battery is formed with a single positive electrode and a single negative electrode, such a battery structure disclosed by Patent Literature 2 cannot realize high power.
Patent Literature 3 discloses a battery which is formed by using a fibrous body having electron conductivity. Patent Literature 3 proposes a processing method of an electric device, in which: a group of first fiber electrodes are arranged into a first layer such that the first fiber electrodes are parallel to each other in the first layer; a group of second fiber electrodes are arranged into a second layer such that the second fiber electrodes are parallel to each other in the second layer; and the second layer is positioned immediately adjacent to the first layer to form electrical connection between the electrodes. This structure prevents occurrence of short-circuiting of a storage battery or a capacitor. Patent Literature 3 also aims at increasing the charging capacity of the battery per unit volume.
Patent Literature 4 discloses a fiber spreading apparatus of an air-flow type, which is capable of spreading an aggregate of fibers, which is to be processed, with high accuracy and efficiency, thereby fabricating a high-quality spread-fiber product. The fiber spreading apparatus disclosed by Patent Literature 4 is intended to uniformly spread an aggregate of fibers in a manner not to cause tangling or cutting of fibers in a fiber bundle.
Patent Literature 5 discloses a method of performing electroplating a bundle of carbon fibers in such a manner that the electroplating is uniformly and continuously performed on each single fiber.
Patent Literature 6 discloses a method of fabricating a metal-oxide-coated carbon fiber. This method allows characteristics of a metal oxide to be maintained, and also allows mechanical characteristics of a carbon fiber, i.e., high strength and high elastic modulus, to be maintained.